1. The Field of the Invention
The present invention relates generally to an intervertebral spacer, and more particularly, but not necessarily entirely, to a interbody spacing system for accomplishing enhanced intervertebral fusion between adjacent vertebral bodies of a human spine.
2. Description of Related Art
The human spine is a complex, sophisticated mechanical system. The vertebrate spine operates as a structural member, providing structural support for the other body parts. A normal human spine is segmented with seven cervical, twelve thoracic and five lumbar segments. The lumbar portion of the spine resides on the sacrum, which is attached to the pelvis. The pelvis is supported by the hips and leg bones. The bony vertebral bodies of the spine are separated by intervertebral discs, which reside sandwiched between the vertebral bodies and operate as joints allowing known degrees of flexion, extension, lateral bending and axial rotation.
The intervertebral disc primarily serves as a mechanical cushion between adjacent vertebral bodies, and permits controlled motions within vertebral segments of the axial skeleton. The disc is a multi-element system, having three basic components: the nucleus pulposus (“nucleus”), the anulus fibrosus (“anulus”) and two vertebral end plates. The end plates are made of thin cartilage overlying a thin layer of hard, cortical bone that attaches to the spongy, richly vascular, cancellous bone of the vertebral body. The plates thereby operate to attach adjacent vertebrae to the disc. In other words, a transitional zone is created by the end plates between the malleable disc and the bony vertebrae.
The anulus of the disc forms the disc perimeter, and is a tough, outer fibrous ring that binds adjacent vertebrae together. The fiber layers of the anulus include fifteen to twenty overlapping plies, which are inserted into the superior and inferior vertebral bodies at roughly a 40 degree angle in both directions. This causes bi-directional torsional resistance, as about half of the angulated fibers will tighten when the vertebrae rotate in either direction.
It is common practice to remove a spinal disc in cases of spinal disc deterioration, disease or spinal injury. The discs sometimes become diseased or damaged such that the intervertebral separation is reduced. Such events cause the height of the disc nucleus to decrease, which in turn causes the anulus to buckle in areas where the laminated plies are loosely bonded. As the overlapping laminated plies of the anulus begin to buckle and separate, either circumferential or radial anular tears may occur. Such disruption to the natural intervertebral separation produces pain, which can be alleviated by removal of the disc and maintenance of the natural separation distance. In cases of chronic back pain resulting from a degenerated or herniated disc, removal of the disc becomes medically necessary.
In some cases, the damaged disc may be replaced with a disc prosthesis intended to duplicate the function of the natural spinal disc. U.S. Pat. No. 4,863,477 (granted Sep. 5, 1989 to Monson) discloses a resilient spinal disc prosthesis intended to replace the resiliency of a natural human spinal disc. U.S. Pat. No. 5,192,326 (granted Mar. 9, 1993 to Bao et al.) teaches a prosthetic nucleus for replacing just the nucleus portion of a human spinal disc.
In other cases it is desired to fuse the adjacent vertebrae together after removal of the disc, sometimes referred to as “intervertebral fusion” or “interbody fusion.”
In cases of intervertebral fusion, it is known to position a spacer centrally within the space where the spinal disc once resided, or to position multiple spacers within that space. Such practices are characterized by certain disadvantages, including a disruption in the natural curvature of the spine. For example, the vertebrae in the lower “lumbar” region of the spine reside in an arch referred to in the medical field as having a sagittal alignment. The sagittal alignment is compromised when adjacent vertebral bodies that were once angled toward each other on their posterior side become fused in a different, less angled orientation relative to one another.
Another disadvantage of known spacing techniques and intervertebral spacers are the additional surgical complications that arise in the use of multiple spacers in a single disc space. In such cases, surgeons will often first perform a posterior surgery to remove the affected disc and affix posterior instrumentation to the posterior side of the vertebrae to hold the posterior portions of the vertebrae in a desired position. Placement of the multiple spacers is often too difficult to accomplish from the posterior side of the patient, at least without causing with undue trauma to the patient, because a surgeon would need to retract the dura nerve as well as the anterior longitudinal ligament, thereby increasing damage, pain and morbidity to the patient. Surgeons have therefore often chosen to turn the patient over after completing the posterior surgical portion, to perform an anterior operative procedure, through the patient's belly, in order to insert multiple spacers between the vertebrae from the anterior side instead of from the posterior side.
U.S. Pat. No. 5,961,554 (granted Oct. 5, 1999 to Janson et al.) illustrates a spacer having a high degree of porosity throughout, for enhanced tissue ingrowth characteristics. This patent does not address the problem of compromising the sagittal alignment, or of increased pain and trauma to the patient by implantation of multiple spacers in a single disk space.
The prior art is thus characterized by several disadvantages that are addressed by the present invention. The present invention minimizes, and in some aspects eliminates, the above-mentioned failures, and other problems, by utilizing the methods and structural features described herein.